(1) Field of the Invention
The present invention relates to a liquid crystal display device.
(2) Related Art Statement
Display devices are a medium for transmitting information visually to people and play an important role for people and groups in the modern information society. The performance of liquid crystal display devices has significantly increased in recent years, and liquid crystal display devices have been adopted as display devices for cellular phones, personal computers and large screen televisions. Liquid crystal display devices are generally formed of a liquid crystal display panel and a backlight (illumination device) that is placed on the rear side of the liquid crystal display panel and illuminates the liquid crystal display panel with light. In the case where a color image is displayed, each pixel is formed of three sub-pixels that correspond to the three primary colors of red, blue and green, for example, and the sub-pixels that correspond to the respective colors are controlled independently so that various colors are reproduced.
Transflective liquid crystal display devices are used as the liquid crystal display devices in mobile devices, such as cellular phones, because they are compatible with various environments for illumination. Transflective liquid crystal display devices have a transmission area and a reflection area within each sub-pixel that forms the display area of the liquid crystal display panel. The transmission area allows a transmissive display to be gained by controlling the amount of transmitted light from the backlight. Meanwhile, reflective displays are gained by controlling the amount of reflected light from the outside in the reflection area. That is to say, transflective liquid crystal display devices primarily provide a transmissive display in a dark environment and a reflective display in a bright environment so that the visibility of display images can be secured, and thus it becomes possible to use the transflective liquid crystal display device in various environments for illumination.
A VA (vertical alignment) system is known as a system for gaining a transflective liquid crystal display device. This has a pair of transparent substrates, a liquid crystal layer sandwiched between these transparent substrates, and a pair of circular polarizers, each of which is provided on the surface of the transparent substrate on the side opposite the liquid crystal layer, and the amount of transmitted light and reflected light is controlled by changing the state of polarization of light that enters into the liquid crystal layer so that an image is displayed. In conventional transflective liquid crystal display devices in a VA system, circular polarizers are used, and the performance of these circular polarizers determines the contrast ratio of the image.
In circular polarizers, a polarizer which functions to absorb a predetermined linearly polarized light component and allows linearly polarized light perpendicular to this component to transmit and a quarter-wave plate are layered on top of each other. In order to increase the performance of the circular polarizer, it becomes necessary to use a quarter-wave plate of which the wavelength dependency on retardation is small, and furthermore to layer the quarter-wave plate and the polarizer precisely on top of each other so that the slow axis of the quarter-wave plate and the absorption axis of the polarizer form a predetermined angle. In actual mass production, however, there is inconsistency in the retardation of the quarter-wave plate and the angle between the slow axis of the quarter-wave plate and the absorption axis of the polarizer, and therefore such a problem arises in that it is difficult to increase the contrast ratio on the transmissive display in transflective liquid crystal display devices having a circular polarizer. That is to say, a system that does not use a circular polarizer is desirable in order to increase the contrast ratio of the transmissive display in a transflective liquid crystal display device in a VA system.
Patent Document 1 describes a liquid crystal display device in a VA system that does not use a circular polarizer. This is provided with another polarization layer in addition to a pair of polarizers that form a liquid crystal display device so that the pixel electrodes and the metal reflective layer are coated. The pair of polarizers are placed in such a manner that the absorption axes are perpendicular to each other, and the absorption axis of the polarization layer coincides with the absorption axis of the polarizer provided on the backlight side. In this case, the transmissive display is of a so-called normally black type where it becomes black (dark) in the case where the driving voltage is zero and becomes bright when a driving voltage is applied. Meanwhile, a reflective display of a normally black type where it becomes a black (dark) display in the case where the driving voltage is zero and a bright display when a driving voltage is applied can be implemented in the same manner as the transmissive display by making the metal reflective layer connected to the pixel electrodes a reflection area. That is to say, a transflective liquid crystal display device in a VA system can be implemented without using a circular polarizer.
Here, Patent Document 1 describes that a TCF (thin crystal film) made by OPTIVA, Inc., is used as the material for the polarization layer. That is to say, a coating type material is used for the polarization layer.    [Patent Document 1] Japanese Unexamined Patent Publication 2005-250430